NULL represents "unknown" or "missing" data, not "empty" or "zero". NULL comparisons work differently than normal values, causing subtle bugs.

NULL is not a value - it's a marker for absence of value. SQL uses three-valued logic: TRUE, FALSE, UNKNOWN.

Key concepts:

• NULL = NULL → UNKNOWN (not TRUE)

• NULL != 5 → UNKNOWN

• WHERE filters only TRUE results

• Aggregates ignore NULL (except COUNT(*))

• Use IS NULL / IS NOT NULL for checks

NULL-related bugs are subtle and common. Every SQL developer must understand NULL behavior to avoid incorrect query results.

NULL represents unknown/missing data. Incorrect NULL handling causes bugs in queries, reports, and applications. Critical for data integrity.